Mason the Interposer

Tuck was six, and he liked to watch Mason work, and one afternoon he watched Mason build a low stub of a wall in the market square that seemed to serve no purpose at all — it held up nothing, it fenced in nothing, it just stood there, half the height of a door, between the fruit stall and the old dog who guarded it.

"That wall's silly," said Tuck. "It doesn't do anything."

"Watch the dog," said Mason.

Tuck watched the dog. The dog, who had spent every market day glaring across the square at anyone who came near the fruit stall, was now glaring at a wall. The stall, on the far side, sat plain and unwatched. The dog could not see past the stone. It was still a very good guard dog. It simply could not guard what it could no longer see.

"You blocked its look," said Tuck, delighted. "You didn't move the dog. You put a wall in the way of its looking."

"I did," said Mason.

"But someone could knock your wall down. It's only little."

"They could," Mason agreed, patting the stub of stone fondly. "And by the time they had, the moment I wanted would already be over. A wall does not need to last forever. It needs to last for the one instant the line must be cut."

Tuck turned this over in his small head. Then he said: "You got in the way on purpose. You interfered."

Mason set down his trowel, which for Mason was a great and rare pause.

"Interfered," he said. "In the way, on purpose. Yes. That is the exact shape of it."

Several winters after, a chunky cheerful rook in a brass-buttoned waistcoat came up the hill road and found Mason laying the last course of a garden wall.

Captain Castle introduced himself and said he ran an academy where children learned the game of sixty-four squares, and that his students had a habit he could not break: when an enemy piece was defended, they gave up on taking it, because a defended piece is safe. "They never think," said Captain Castle, "to ask whether they might simply step something into the line between the guard and its charge — even a piece the guard could capture — and cut the defence for the one move they need."

Mason smoothed the last stone flat.

"I have spent my life," he said, "putting walls across lines that others could not even see. A board of sixty-four squares must be full of lines. I should like to cut a few."

The chessboard, Mason discovered, was a town drawn in straight grey lines.

He met his first class on a cold bright morning. Captain Castle set a position: a black rook stood ready to be captured, but it was defended along a rank by a black bishop, so the children shook their heads and looked elsewhere.

"Can't take the rook," said a fox in a green scarf. "The bishop's guarding it, all the way down the row."

"Is it?" said Mason. He pointed, slowly, at the empty squares between the bishop and the rook — the line along which the bishop was looking. "The bishop guards the rook along this line. Watch what happens if I stand one of my own pieces in the middle of it." He moved a knight into the line, square in the bishop's path. "Now the bishop cannot see the rook at all. The wall is up. The line is cut."

"But — they can just take your knight!" said the fox.

"They can," said Mason, calmly. "But not this move. This move, they must choose: recapture my knight and let me have the rook, or save the rook and let me keep the plan. For one instant, the wall stands — and one instant is all a wall must last." He lifted the undefended rook off the board. "The bishop was a fine guard. It simply could not guard through a wall."

The fox in the green scarf wrote a guard cannot guard through a wall — even a wall that falls next move in her notebook, and sat very still. Captain Castle nodded from the doorway.